Frame memory circuit

ABSTRACT

A memory circuit ( 14 ) having features specifically adapted to permit the memory circuit ( 14 ) to serve as a video frame memory is disclosed. The memory circuit ( 14 ) contains a dynamic random access memory array ( 24 ) with buffers ( 18, 20 ) on input and output data ports ( 22 ) thereof to permit asychronious read, write and refresh accesses to the memory array ( 24 ). The memory circuit ( 14 ) is accessed both serially and randomly. An address generator ( 28 ) contains an address buffer register ( 36 ) which stores a random access address and an address sequencer ( 40 ) which provides a stream of addresses to the memory array ( 24 ). An initial address for the stream of addresses is the random access address stored in the address buffer register ( 36 ).

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates in general to digital memory circuits. Specifically, the present invention relates to digital memory circuits which have particular advantages when used in connection with video applications.

BACKGROUND OF THE INVENTION

[0002] Digital TV, VCR, and related video applications often utilize a frame or field memory that stores pixels which together represent an entire frame of video. Such a frame memory is used in producing a variety of special effects, such as frame freezing, zoom, pan, split screen monitoring, and the like. Although a frame memory may be constructed using conventional discrete integrated circuits, such a frame memory is relatively expensive, dissipates an undesirably large amount of power, and occupies an undesirably large amount of space. When such a frame memory is targeted for use in a commercial product, these problems are major ones. Accordingly, a single integrated circuit, either alone or in combination with as few other integrated circuits as possible, improves upon a frame memory which has been constructed from conventional discrete integrated circuits.

[0003] Prior art integrated circuit devices have attempted to address the frame memory problem. However, such devices fail to provide an architecture which adequately addresses video application needs. For example, devices which include only a few of the typically needed frame memory functions may be used in providing a wide variety of special effects. However, they must be combined with such a large quantity of conventional discrete integrated circuits that little improvement results over constructing a frame memory entirely from conventional discrete integrated circuits. On the other hand, a conventional frame memory integrated circuit may include a random access memory with complete on-chip address calculation. A video application which utilizes such a frame memory accesses the entire frame memory serially. Thus, frame freeze and split screen monitoring special effects are supported. However, zoom and pan functions are either impossible or impractical using such a device.

[0004] Accordingly, the industry feels a need for a frame memory integrated circuit which optimizes circuit architecture to accommodate a wide variety of special effects without requiring a large quantity of surrounding integrated circuits.

SUMMARY OF THE INVENTION

[0005] Accordingly, it is an advantage of the present invention that a frame memory circuit is provided which permits limited random access. Consequently, a device constructed according to the teachings of the present invention may be efficiently used to perform a wide variety of special effect video applications.

[0006] Another advantage of the present invention is that a memory circuit is provided which includes a variety of address calculation modes. Thus, a portion of the address calculations for certain special effect functions may be transferred to the memory circuit, and a video application which utilizes such a memory circuit need not allocate processing power to such calculations.

[0007] The above advantages of the present invention are carried out in one form by a memory circuit which stores and provides streams of data. This memory circuit supports both serial access and random access. A data input of a random access memory array couples to a data buffer so that the data buffer may synchronize operation of the memory array with the streams of data. An address input of the random access memory array couples to an address sequencer which generates a sequence of memory addresses that are successively applied to the memory array. An address buffer register also couples to the address sequencer. The address buffer register supplies a random access address to the address sequencer to initialize the sequence of memory addresses supplied by the address sequencer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the accompanying drawings, in which like reference numbers indicate like features throughout the drawings, and wherein:

[0009]FIG. 1 illustrates a frame of a video display screen with which the present invention may be used;

[0010]FIG. 2 shows a block diagram of a memory circuit built according to the teachings of the present invention;

[0011]FIG. 3 shows a block diagram of a first alternate embodiment of an address generator portion of a memory circuit built according to the teachings of the present invention;

[0012]FIG. 4 shows a block diagram of a second alternate embodiment of an address generator portion of a memory circuit built according to the teachings of the present invention; and

[0013]FIG. 5 shows a block diagram of an address sequencer utilized by the address generator portion of a memory circuit built according to the teachings of the present invention.

DETAILED DESCRIPTION

[0014]FIG. 1 illustrates a video frame 10, such as may appear on a TV tube or other video display terminal. Although frame 10 may appear as a continuous video picture to a viewer, frame 10 may be electrically represented as a multiplicity of digitized pixels 12. Each one of pixels 12 defines parameters, such as color and relative intensity, for one of a multiplicity of very small areas within the picture of frame 10. Accordingly, frame 10 may contain a relatively large number of pixels 12. For example, a frame containing 488 columns of pixels 12 by 488 rows of pixels 12 has a total of 238,144 pixels per frame.

[0015] Pixels 12 are typically transmitted or otherwise processed in a predetermined order to preserve the spatial relationships between pixels 12. For example, in a conventional raster scan application, pixels 12 may be transmitted to a memory device or a video display in successive order beginning with a pixel 12 a, that represents the pixel 12 in the first column of the first row of frame 10, and continuing in successive order to a pixel 12 b, which represents the pixel 12 in the last column of the first row of frame 10. Immediately following the transmission of pixel 12 b and sync information (not shown), a pixel 12 c, which represents the pixel 12 in the first column of the second row, may be transmitted followed in successive order by the remaining pixels 12 contained in the second row of frame 10. Transmission of pixels 12 continues in this fashion until a pixel 12 d, which represents the pixel 12 in the last column of the last row of frame 10, has been transmitted. Thus, any processing device which knows the timing relationship between a pixel 12 and beginning pixel 12 a also knows or can easily calculate the spatial location of such pixel 12 within frame 10.

[0016] A digital TV, VCR, or the like may contain a large frame or field memory which is capable of storing all of pixels 12 within frame 10. Pixels 12 collectively appear as a serial data stream to the frame memory. Except for special effects, the relative order of pixels 12 in this serial data stream must generally be preserved when read from the frame memory to preserve the spatial relationships between pixels 12. However, various special effects do not require this preserved order, and valuable computation time may be wasted by precisely preserving the order of pixels 12 as pixels 12 are being read from the frame memory.

[0017] One such special effect is a zoom effect wherein a small portion of a frame is expanded to fill an entire video display. For example, if frame 10 in FIG. 1 represents an entire video display, then an area within frame 10 bounded by rows i and j and columns m and n is expanded in a zoom special effect to fill the entire frame 10. Thus, in the zoom special effect all of pixels 12 residing within frame 10 outside of the area bounded by rows i and j and columns m and n are inactive and may be discarded. In other words, these inactive ones of pixels 12 need not be stored or read from the frame memory. Consequently, the pixel 12 located at column m and row i will be utilized as first pixel 12 a in the zoom special effect. Active pixels 12 may be duplicated to complete an entire row of frame 10, and rows may be duplicated to complete the vertical component of the zoom effect.

[0018] In a split screen special effect, an entire frame 10 may be shrunk into a small area of a screen, such as that bounded by row j and the last row of frame 10, and column n and the last column of frame 10. This special effect is accomplished by utilizing only one of pixels 12 out of each of a predetermined number of pixels 12 from an entire frame 10 of pixels 12, and ignoring the intervening inactive ones of pixels 12 (ie. skipping pixels). For the example depicted in FIG. 1, the shrunken frame is formed using only pixels 12 from one of every three columns and one of every three rows of frame 10.

[0019] The present invention provides a memory circuit which serves as a frame memory and permits these and other special effects to be performed efficiently. FIG. 2 shows a block diagram of a memory circuit 14 built according to the teachings of the present invention. In general, the preferred embodiment of memory circuit 14 represents a single chip integrated circuit that contains 2²⁰ or 1,048,576 bits of memory storage organized as 262,144 four bit wide words. Accordingly, a sufficient quantity of words are provided to buffer or store an entire 488×488 frame of pixels 12 (see FIG. 1). If more than four bits of precision are required to accurately describe each pixel, then additional ones of memory circuit 14 may be used to store such additional bits.

[0020] Memory circuit 14 generally operates in a serial access mode but has particular features which permit random access of memory circuit 14 on a limited scale. Those skilled in the art will understand that serial access refers to a mode of storing and reading data in which data must be read out from a memory in the same order in which it was stored into the memory. Furthermore, random access refers to the ability to write, read, or otherwise access any location in a memory array by suppying a unique address which corresponds to such memory location.

[0021] Specifically, memory circuit 14 includes a serial pixel data input 16 a, which in the preferred embodiment supplies four bits of data. Serial pixel data input 16 a couples to an input port of a write serial latch 18 a, and an output port of write serial latch 18 a couples to an input port of a write register 20 a. An output port of write register 20 a couples to a data input port 22 a of a memory array 24. In the preferred embodiment, memory array 24 is a dynamic random access memory (DRAM) array containing 2¹⁸ or 262,144 four bit memory locations. A data output port 22 b of memory array 24 couples to a data input port of a read register 20 b, and a data output port of read register 20 b couples to a data input port of a read serial latch 18 b. A data output port of read serial latch 18 b couples to a serial pixel data output 16 b, which in the preferred embodiment provides four bits of data.

[0022] A serial write clock terminal 26 a couples to a write address generator 28 a, an arbitration and control circuit 30, and a clock input of write serial latch 18 a. Similarly, a serial read clock terminal 26 b couples to a read address generator 28 b, arbitration and control circuit 30, and a clock input of read serial latch 18 b. A refresh address and timing circuit 32 has an output which couples to an input of arbitration and control circuit 30, and outputs from arbitration and control circuit 30 couple to a clock input of write register 20 a, a clock input of read register 20 b, a control input of memory array 24, and an address input of memory array 24.

[0023] As shown in FIG. 2, address generators 28 a and 28 b are structurally similar to one another in the preferred embodiment. Thus, a write control data terminal 34 a couples to a serial data input of an address buffer register 36 a in write address generator 28 a. A read control data terminal 34 b couples to a serial data input of an address buffer register 36 b in read address generator 28 b. Likewise, a write control strobe terminal 38 a couples to a clock input of address buffer register 36 a, and a read control strobe terminal 38 b couples to a clock input of address buffer register 36 b. A data output of address buffer register 36 a couples to a data input of an address sequencer 40 a, and a data output of address buffer register 36 b couples to a data input of an address sequencer 40 b. A write reset terminal 42 a couples to a clear input of address sequencer 40 a, and a write transfer terminal 44 a couples to a preset input of address sequencer 40 a. A read reset terminal 42 b couples to a clear input of address sequencer 40 b, and a read transfer terminal 44 b couples to a preset input of address sequencer 40 b. Terminal 26 a couples to a clock input of address sequencer 40 a within address generator 28 a, and terminal 26 b couples to a clock input of address sequencer 40 b within address generator 28 b. An output 46 a of address sequencer 40 a presents the output signal from address generator 28 a and couples to an input of arbitration and control circuit 30. Likewise, an output 46 b of address sequencer 40 b presents the output signal from address generator 20 b and couples to arbitration and control circuit 30. Memory circuit 14 may be provided in a 20 pin integrated circuit package.

[0024] As discussed above, memory circuit 14 may be operated in either a serial or a limited random access mode. In addition, the storing or writing of data into memory circuit 14 may occur asynchroniously with the reading or providing of data from memory circuit 14. Memory circuit 14 may be written into serially by activating write reset signal on terminal 42 a to clear address sequencer 40 a. Then, a four bit wide stream of serial data may be stored in memory circuit 14 by applying the four bit data nibbles at data input 16 a while asserting a serial write clock signal at terminal 26 a. One assertion of the serial write clock signal causes write serial latch 18 a to temporarily store or buffer one four bit data nibble. Write serial latch 18 a operates as a four bit wide shift register. Thus, subsequent four bit nibbles from the data stream of serial pixel data applied at data input 16 a are shifted into serial latch 18 a upon subsequent assertions of the serial write clock signal.

[0025] In addition, each assertion of the serial write clock signal causes address sequencer 40 a of write address generator 28 a to supply a new a random access address to arbitration and control circuit 30. In other words, address sequencer 40 a provides a stream of addresses to arbitration and control circuit 30 which corresponds to the stream of data being stored in write serial latch 18 a.

[0026] Arbitration and control circuit 30 receives addresses from address generators 28 a-28 b and refresh address and timing circuit 32. Circuit 30 monitors these inputs and various timing signals to decide which of the addresses provided on these inputs should be transferred to memory array 24. Arbitration and control circuit 30 includes conventional logic circuits for controlling the timing operation of the dynamic memories which comprise memory array 24. Thus, arbitration and control circuit 30 passes an address generated by address generator 28 a onto memory array 24 so that data may be written into memory array 24, but a delay may occur due to refresh operations or read accesses of memory array 24. Accordingly, arbitration and control circuit 30 may additionally contain storage devices so that addresses generated by address generators 28 a-28 b are not lost when immediate access to memory array 24 is blocked. When arbitration and control circuit 30 identifies a time at which the serial pixel data may be written into memory array 24, such data is transferred from write serial latch 18 a into write register 20 a and then written into memory array 24. Accordingly, write serial latch 18 a and write register 20 a together represent a double buffering scheme which permits asynchronious operation of memory array 24 with the storing of serial pixel data into memory circuit 14.

[0027] The reading of data from memory array 24 occurs in a manner similar to that described above for the storing of data into memory array 24. Thus, an address generated by address generator 28 b is transferred through arbitration and control circuit 30 at an appropriate time to cause data from memory array 24 to be read into read register 20 b. Thereafter, this data is transferred into read serial latch 18 b so that such data may be provided at data output terminal 16 b through the application of a serial read clock signal at terminal 26 b. Serial data is provided at output 16 b asynchroniously with the operation of memory array 24 and asynchroniously with the storing of serial pixel data into memory circuit 14 at terminal 16 a.

[0028] The limited random access feature of memory circuit 14 is provided through address generators 28 a-28 b. In the embodiment of memory circuit 14 shown in FIG. 2, write address generator 28 a and read address generator 28 b are structurally and operationally identical, except that write address generator 28 a provides write addresses while read address generator 28 b provides read addresses. Accordingly, both of address generators 28 a-28 b are described below by reference only to write address generator 28 a. Those skilled in the art will recognize that read address generator 28 b operates identically in the preferred embodiment.

[0029] A random access address may be serially loaded into address buffer register 36 a by applying such address to control data terminal 34 a in a sequential manner and activating a control strobe signal applied at terminal 38 a when valid data appear at terminal 34 a. Thus, in the embodiment shown in FIG. 2, address buffer register 36 a represents a serial shift register. The use of a serial shift register conserves the number of external pins needed for constructing memory circuit 14 in an integrated circuit when compared to a parallel loaded register. After the random access address has been entered into address buffer register 36 a, it may be transferred to data sequencer 40 a by the application of a write transfer signal at terminal 44 a. In the preferred embodiments of the present invention, address sequencer 40 a may represent a presetable, binary counter or other presetable sequencing circuit. Thus, the transferred address initiates a sequence of addresses which are subsequently generated by address generator 28 a. If address sequencer 40 a represents a binary counter, then subsequent addresses will increment or decrement starting with this preset value.

[0030] If memory array 24 contains 2¹⁸ four bit words of memory, then address buffer register 36 a may advantageously represent an 18 bit register, and address sequencer 40 a may represent an 18 bit counter, or other sequencing circuit. On the other hand, address buffer register 36 a and address sequencer 40 a may contain fewer bits, such as nine bits for example. In the nine bit situation, the random access address provided by address buffer register 36 a could access the beginning of memory pages or rows wherein each page or row contains 2⁹ or 512 words of memory.

[0031] The inclusion of address buffer register 36 a to provide a limited random access feature permits memory circuit 14 to be efficiently utilized in a zoom special effect. For example, a zoom effect may be accomplished by writing an entire frame of memory into memory array 24 using a serial access mode. A beginning pixel address, such as the address of a pixel located at row i column m in FIG. 1, may then be loaded into read address buffer register 36 b and transferred to address sequencer 40 b. A first row, such as row i, of the portion of frame 10 which is to be expanded into an entire frame may then be read from memory array 24 in a serial or sequential mode until a pixel corresponding to, for example, row i, column n appears at output terminal 16 b. A row may be repeated as often as necessary to achieve vertical zoom by transferring the random access address from address buffer register 36 b to address sequencer 40 b. An address corresponding to the pixel located at row i+1 and column m may then be loaded into address buffer register 36 b and transferred to address sequencer 40 b. This process continues until a final pixel for the frame to be expanded has been output from memory array 24. Due to this feature, a video system need not start accesses of memory circuit 12 at an initial address, such as pixel 12 a (shown in FIG. 1) and access inactive pixels stored within memory array 24. Faster operation results.

[0032] The present invention contemplates alternate embodiments of address generators 28 a-28 b. A first alternate embodiment of address generators 28 a-28 b is shown in FIG. 3. FIG. 3 shows only one of address generators 28. The address generator 28 shown in FIG. 3 may serve as either write address generator 28 a or read address generator 28 b (see FIG. 2).

[0033] In this first alternate embodiment of an address generator 28, address buffer register 36 may be loaded both serially and in parallel. Thus, control data terminal 34, which may represent either write control data terminal 34 a or read control data terminal 34 b, as discussed above in connection with FIG. 2, couples to the serial data input of address buffer register 36. Control strobe terminal 38 couples to the serial clock input of address buffer register 36 and a serial clock input of an address offset register 48. The parallel data output of address buffer register 36 couples to a first input of an adder 50 and the data input of address sequencer 40. A parallel data output of address offset register 46 couples to a second input of adder 50. An output of adder 50 couples to a parallel data input of address buffer register 36, and transfer terminal 44 couples to a parallel clock input of address buffer 36 and the preset input of address sequencer 40. A most significant bit from the parallel data output or a serial output bit, of address buffer register 36 couples to a serial data input of address offset register 48. Serial clock terminal 26 couples to the clock input of address sequencer 40, and reset terminal 42 couples to a clear input of address sequencer 40. A data output of address sequencer 40 couples to address generator output 46.

[0034] Address buffer register 36 and address sequencer 40 operate in this first alternate embodiment similarly to their above-described operation in connection with address generator 28 a-28 b of FIG. 2. However, in this first alternate embodiment, the control data provided at terminal 34 is used to load both address buffer register 36 and address offset register 48. Thus, additional bits of control data are loaded into memory circuit 14 without requiring additional integrated circuit pins. Moreover, a most significant bit, or a serial output bit 51, from address offset register 48 may advantageously be routed to the control data input for the other one of read and write address generators 28 a and 28 b (see FIG. 1). In addition, the control strobe signal applied at terminal 38 may be routed to the other one of control strobe terminals 38 a and 38 b of FIG. 2. These two connections between address generators 28 a and 28 b eliminate two integrated circuit pins from the structure shown in FIG. 2.

[0035] In this first alternate embodiment of the present invention, the control data contained in address offset register 48 is added to a current initial address value contained in address buffer register 36 to provide a new initializing random access address value. This new initializing value is loaded into address buffer register 36 when the current address value is transferred into address sequencer 40.

[0036] Referring additionally to FIG. 1, the first alternate embodiment of the present invention may be advantageous in performing, for example, the zoom special effect. Thus, the address offset value loaded into address offset register 48 may represent the quantity of inactive pixels occurring between column n of one row and column m of the next row. At the end of each frame row a transfer signal may be asserted on terminal 44, and the random access address of the next active pixel, corresponding to column n of the next row, is automatically calculated and stored in address buffer register 36 to initiate another sequence of sequential accesses to memory circuit 16. Complexity of a video system employing memory circuit 14 decreases because components external to memory circuit 14 need not calculate this address.

[0037] A second alternate embodiment of address generators 28 a-28 b from FIG. 2 is shown in FIG. 4. The FIG. 4 embodiment illustrates that random access addresses may be loaded into address buffer register 36 in a parallel fashion, which may be more compatible with conventional microprocessor integrated circuits. However, the number of integrated circuit pins needed to implement this embodiment increases over the embodiments discussed above in connection with FIGS. 2 and 3. In addition, FIG. 4 shows the inclusion of an alternate address buffer register 52 in addition to address buffer register 36. Specifically, control data terminals 34 may advantageously provide an eight bit microprocessor data bus which couples to data inputs of individual eight bit portions 54 a, 54 b, and 54 c of address buffer register 36. In addition, control data terminals 34 couple to data inputs of individual eight bit portions 56 a, 56 b, and 56 c of alternate address buffer register 52. Data outputs of individual portions 54 a-54 c together form a 24 bit bus which couples to a first data input of a multiplexer 58. Likewise, data outputs of individual portions 56 a-56 c form a 24 bit bus which couples to a second data input of multiplexer 58. A data output of multiplexer 58 couples to a data input of a binary counter which serves as address sequencer 40 in this second alternate embodiment. Of course, those skilled in the art will recognize that the number of subregisters included within address buffer register 36 and alternate address buffer register 52 and the number of bits contained within the buses described above are subject to a substantial variation in accordance with specific application requirements.

[0038] In addition, microprocessor address input terminals 60 a, 60 b, and 60 c, couple to address inputs of a decoder 62 and an address input terminal 60 d couples to an enable input of decoder 62. The control strobe terminal 38, discussed above, couples to an enable input of decoder 62. Outputs 01-06 of decoder 62 couple to clock inputs of individual address buffer register portions 54 a-54 c and clock inputs of individual alternate address buffer register portions 56 a-56 c, respectively. An output 07 from decoder 62 couples to a clock input of a flip flop 64 which is configured to toggle upon the activation of the clock input. An output of flip flop 64 couples to a select input of multiplexer 58. An output 08 of decoder 62 couples to a preset input of binary counter 40. The serial clock 26 couples to a clock input of binary counter 40, and reset terminal 42 couples to a clear input of flip flop 64 and a clear input of binary counter 40. An output of binary counter 40 couples to output 46 of address generator 28.

[0039] In this second alternate embodiment of address generator 28, one initializing random access address may be stored in address register 36 while an alternate initializing random access address is stored in alternate address buffer register 52. A microprocessor (not shown) may store these addresses in memory circuit 14 through conventional memory or I/O write operations to addresses specified by signals applied on terminals 60 a-60 c. An address input bit applied at terminal 60 d may advantageously distinguish between a write address generator 28 a and a read address generator 28 b (see FIG. 1). By applying an active signal to reset terminal 42, flip flop 64 and binary counter 40 may be initialized to a cleared state. At this point, address generator 28 operates substantially as described above in connection with FIG. 2. However, an alternate random access address stored in alternate address buffer 52 may selectively preset binary counter 40. A microprocessor write operation which toggles flip flop 54, followed by a microprocessor write operation that transfers data into binary counter 40, presets binary counter 40 with an alternate random access address. Flip flop 64 may be toggled by performing a write operation to the address which activates output 07 of decoder 62. A transfer operation from the selected one of address buffer registers 36 and 52 occurs by writing to the address which activates the output 08 of decoder 62.

[0040] Alternate address buffer register 52 may advantageously be used by a video system to efficiently buffer a line within a frame of data. Since memory circuit 14 of the preferred embodiment contains a sufficient quantity of memory to accommodate 2¹⁸ or 262,144 pixels, memory circuit 14 has unused memory locations when used to store a single frame of data which contains, for example, 480 pixel columns by 480 pixel rows. Accordingly, a random access address in this unused portion of memory may be loaded in alternate address buffer register 52. A single line of a frame may be efficiently stored in memory circuit 14 by transferring this alternate address value to binary counter 40, then sequentially storing such line of pixels into the otherwise unused portion of memory circuit 14.

[0041] In addition, the present invention contemplates alternative embodiments for address sequencer 40. As shown in FIG. 4, address sequencer 40 may represent a conventional presetable, clearable, binary counter. Such circuits are well known to those skilled in the art and need not be described in detail herein. However, address sequencer 40 may alternatively represent a circuit which increments or decrements by a variable step value which may differ from the value of one. Such a circuit is shown in FIG. 5.

[0042] Accordingly, in FIG. 5 the address sequencer data input couples to a first input of a multiplexer 66, the address sequencer's preset terminal couples to a select input of multiplexer 66. An output of multiplexer 66 couples to a data input of a register 68, and the clock input of address sequencer 40 couples to a clock input of register 68. Likewise, the reset terminal 42 couples to a clear input of register 68. A data output of register 68 provides the data output of address sequencer 40 and additionally couples to a first input of an adder 70. An output of adder 70 couples to a second input of multiplexer 66. The control data terminals 34, discussed above in connection with FIGS. 2-4, couple to a data input of a register 72. Additionally, the control strobe terminal 38, discussed above in connection with FIGS. 2-4, couples to a clock input of register 72. A data output of register 72 couples to a second input of adder 70.

[0043] In this FIG. 5 embodiment of address sequencer 40, register 72 may represent either a parallel or a serially loaded register, as discussed above in connection with FIGS. 2-4. Additionally, if register 72 represents a serially loaded register, then register 72 may represent one register out of many coupled together in a long chain of serially loaded registers, as discussed above in connection with FIG. 3. The data loaded into register 72 is intended to represent a increment step by which address sequencer 40 generates successive addresses at output 46 of address generator 28. A current output of address sequencer 40 is added to this step increment value in adder 70, and routed through multiplexer 66 back to register 68. Thus, a subsequent address generated by address sequencer 40 equals the previous address plus the address step increment contained in register 72. This address step increment need not equal the value of one but may equal any positive or negative value. Furthermore, if the number of bits contained in the buses which couple register 72, adder 70, multiplexer 66, and register 68 together is greater than the number of bits provided at the output of address sequencer 40, then subsequent addresses may be incremented in fractional steps.

[0044] Address sequencer 40 may be preset, or initialized, with a random access address by applying an active signal on the preset terminal, supplying data at the data input terminals, and clocking the clock signal of address sequencer 40. Thus, this initializing random access value is loaded directly into register 68. In addition, address sequencer 40 may be cleared, or reset, by applying a reset signal to the clear input terminal.

[0045] Referring additionally to FIG. 1, the address sequencer 40 depicted in FIG. 5 is useful in performing the split screen special effect where an entire frame is displayed in only a small portion of a video screen, such as the lower right hand portion shown in FIG. 1. With this special effect, if memory circuit 14 has every pixel 12 of a frame 10 stored therein, then only one out of every group of a predetermined number of stored pixels is active in constructing the shrunken screen. Address sequencer 40 shown in FIG. 5 allows memory circuit 14 to provide only the active pixels by supplying a sequence of addresses which omits inactive pixel addresses.

[0046] In summary, the present invention provides a memory circuit which allows a video system to efficiently perform special effects. Specifically, the inclusion of various limited random accessing features allows memory circuit 14 to store and/or provide only active pixels for a given special effect and not inactive pixels. Consequently, active pixels may be retrieved from memory circuit 14 much quicker than occurs with the use of prior art frame memory circuits.

[0047] The foregoing description uses preferred embodiments to illustrate the present invention. However, those skilled in the art will recognize that changes and modifications may be made in these embodiments without departing from the scope of the present invention. For example, read address generator 28 b need not precisely resemble write address generator 28 a. Additionally, although the embodiments depicted in FIGS. 3-5 are mentioned above as being alternative embodiments, nothing prevents one skilled in the art from combining the teachings from more than one of these alternate embodiments into a single frame memory circuit 14. Moreover, those skilled in the art will recognize that additional address processing cap,abilities may be built into frame memory circuit 14. Such additional address processing capabilities may include the addition of a signal which indicates the end of a frame line, a signal which indicates the end of a frame, and the automatic transferring of random access addresses to an address sequencer upon the occurrence of the end of line and end of frame signals. Furthermore, although specific frame and memory array dimensions have been presented herein to aid in teaching the present invention, it is intended that the present invention not be limited to any particular dimensions. These and other modifications obvious to those skilled in the art are intended to be included within the scope of the present invention. 

What is claimed is:
 1. A memory circuit for storing and providing streams of data, said memory circuit accommodating both serial access and random access, and said memory circuit comprising: a random access memory array having an address input and a data port; a data buffer having a data port coupled to said memory array data port, said data buffer synchronizing operation of said memory array to the streams of data; an address sequencer having a data input and having an output coupled to said memory array address input, said address sequencer generating a sequence of memory addresses to be successively applied to said memory array; and an address buffer register having an output coupled to said address sequencer data input, said address buffer register supplying a random access address that initializes the sequence of memory addresses generated by said address sequencer.
 2. A memory circuit as claimed in claim 1 wherein said address buffer register comprises a serially loaded shift register.
 3. A memory circuit as claimed in claim 1 additionally comprising a terminal coupled to said address sequencer and adapted to receive a signal which causes data contained in said address buffer register to transfer to said address sequencer.
 4. A memory circuit as claimed in claim 1 wherein said memory array, data buffer, address sequencer, and address buffer register are included within a single integrated circuit.
 5. A memory circuit as claimed in claim 1 wherein said address sequencer comprises a binary counter having a data input coupled to the output of said address buffer register and an output coupled to the address input of said memory array.
 6. A memory circuit as claimed in claim 1 wherein said address sequencer comprises: a first register having a data input coupled to a node which serves as said address sequencer data input and an output which serves as said address sequencer output; a second register having an output, said second register being for storing an increment step value; and an adder having a first input coupled said first register output, a second input coupled to said second register output, and an output coupled to said first register data input.
 7. A memory circuit as claimed in claim 1 wherein said data buffer synchronizes operation of said memory array to the data stream being stored into said memory array, said address sequencer generates memory addresses at which the stored data stream is written into said memory array, and said memory circuit additionally comprises: a second data buffer having a data port coupled to said memory array data port, said second data buffer being for synchronizing operation of said memory array to the data stream provided by said memory circuit; a second address sequencer having an output coupled to said memory array address input and a data input, said second address sequencer generating a sequence of memory addresses to be applied to said memory array for reading the provided data stream from said memory array; and a second address buffer register having an output coupled to said second address generator data input, said second address buffer register supplying a random access address that initializes the sequence of memory addresses generated by said second address sequencer.
 8. A memory circuit as claimed in claim 1 additionally comprising: an address offset register having an output, said address offset register being for storing address offset data; and an adder having a first input coupled to said address buffer register output, a second input coupled to said address offset register output, and an output coupled to a data input of said address buffer register, said adder providing a random access address representing a sum of a past random access address and said address offset data.
 9. A memory circuit as claimed in claim 1 additionally comprising an alternate address buffer register having an output coupled to said address sequencer data input, said alternate address buffer register supplying an alternate random access address that initializes an alternate sequence of memory addresses generated by said address sequencer.
 10. An integrated memory circuit for storing and providing streams of data, said integrated memory circuit accommodating serial access and limited random access, and said integrated memory circuit comprising: a random access memory array having an address input, a data input port, and a data output port; a first data buffer having a data port coupled to said memory array data input port, said first data buffer synchronizing operation of said memory array to the stored stream of data; a second data buffer having a data port coupled to said memory array data output port, said second data buffer synchronizing operation of said memory array to the provided stream of data; and first and second address generators wherein said first address generator generates addresses used for writing the stored data stream into said memory array, said second address generator generates addresses used for reading the provided data stream from said memory array, and each of said first and second address generators comprises: a binary counter having an output coupled to said memory array address input and a data input, said binary counter being for counting memory addresses to be applied to said memory array; and a serially loaded address buffer register having an output coupled to said binary counter data input, said address buffer register being for supplying an initial random access memory address which starts the count of said binary counter.
 11. A memory circuit as claimed in claim 10 wherein each of said first and second address generators additionally comprises: an address offset register having an output, said address offset register storing address offset data; and an adder having a first input coupled to said address buffer register output, a second input coupled to said address offset register output, and an output coupled to a data input of said address buffer register, said adder providing a sum of a past random access address and said address offset data to said address buffer register.
 12. A memory circuit as claimed in claim 10 wherein each of said first and second address generators additionally comprises an alternate buffer register having an output coupled to said binary counter data input, said alternate address buffer register supplying an alternate initial random access memory address which is counted by said binary counter.
 13. A method of storing and providing streams of data using random access memory array, said method comprising the steps of: bufferinq the streams of data into and out from the memory array so that the stored and provided data streams occur asynchroniously with operation of the memory array; generating a random access address; and generating a sequence of addresses initialized with said random access address said addresses being successively applied to the random access memory array.
 14. A method as claimed in claim 13 wherein said generating a random access address step comprises one step of serially loading a register with the random access address.
 15. A method as claimed in claim 13 wherein said generating a sequence step comprises the step of counting successive data items within the streams of data to generate addresses for successive application to the random access memory array.
 16. A method as claimed in claim 13 wherein said generating a sequence step generates addresses for writing the stored data stream into the array, and said method additionally comprises the step of: generating a second sequence of addresses which are successively applied to the random access memory array for reading the provided data stream from the memory array; and supplying, to said generating a second sequence step, a random access address which initiates the successively applied sequence of addresses.
 17. A method as claimed in claim 13 additionally comprising the steps of: providing an address offset value; and adding the address offset value to the random access address to generate a second random access address.
 18. A method as claimed in claim 13 additionally comprising the step of supplying, to said generating a sequence step, a second random access address which initiates a second successively applied sequence of addresses.
 19. A method as claimed in claim 13 wherein said generating a sequence step comprises the steps of: providing an increment step value; and adding the increment step value to a current address from the sequence of addresses to produce a next address in the sequence of addresses. 